Because a medium called music is recognized and expressed in a vague way, it is generally difficult for a user with little knowledge of music to cause a computer to compose or perform a musical piece as he/she desires. In order to realize a musical system that can be manipulated by a user with little knowledge of music, two things are important: (1) how to manipulate music, and (2) how to reflect a user's intention in the music. One thing to note is that increasing the abstraction level of an object to be operated makes it easier to manipulate music but may make it more difficult to reflect a user's intention in the music.
For example, musical score editors and sequencers (Non-Patent Document 1) are commercially available. However, such editors and sequencers can manipulate only surface structures of music with low vagueness, such as notes, rests, and chord names. Meanwhile, Non-Patent Document 2 (http://www.apple.com/jp/ilife/garageband/) discloses a system that allows composing a musical piece just by simple operations, such as combining some of a large number of loop materials prepared in advance by the system.
Non-Patent Document 3 proposes a technique for morphing two contents using a relative pseudo-complement.    [Non-Patent Document 1] Tenpei Sato, “Computer Music Super Beginners' Manual”, Softbank Creative Corporation, 1997    [Non-Patent Document 2] http://www.apple.com/jp/ilife/garageband/    [Non-Patent Document 3] Keiji Hirata and Satoshi Tojo, “Formalization of Media Design Operations Using Relative Pseudo-Complement”, 19th Annual Conference of Japanese Society for Artificial Intelligence, 2B3-08, 2005